Finsterwalder



July 12, 1955 u. FINSTERWALDER 2,712,750

METHOD OF CONSTRUCTING REINFORCED CONCRETE BRIDGE Filed April ll, 1949 3Shee'tS--SheelI l l' il l'neys July l2, 1955 u. FlNsTERwALDl-:R

METHOD OF CONSTRUCTING REINFORCED CONCRETE BRIDGE I 5 Sheets-Sheet 2Filed April ll. 1949 /n ven for u, s n .fr

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July 12, 1955 u.F1NsTERwALDER 2,712,750

METHOD OF' CONSTRUCTING REINFORCED CONCRETE BRIDGE Filed April 1l, 19495 Sheets-Sheet 5 In vemor United States Patent 2,712,756 l THGD i323CGNSRUCTING REINFQRCEB CNCRETE BRIDGE Ulrich Finsterwaldet', Munich,Germany Application April l1, i949, Serial No. 86,668 Claims priority,application France April 22, 1948 2 Claims. (Cl. 72-8) The presentinvention relates to a truss bridge of re-enforced concrete and to amethod of its construction.

Prior to this invention it was the practice, as a rule, to constructbridges of re-enforced concrete extending over a plurality of spans as acontinuous beam in form of a plate provided with webs. limited becauseof the extreme weight or such webs with large vertical dimensions. Whileit is true that the weight may be somewhat, reduced by a design of thebeams as box-shaped elements better adapted to sustain bending torques,reduction of the thickness of the beams, structures of that kind areuneconomical for spans of l5() feet or more because of their excessiveweight.

Moreover, steps have been taken to increase the Vertical dimension ofthe within the region of the middle pillars to thereby increase the zoneof the negative bending torques and to decrease the zone of the positivebending torques intermediate the pillars. This development, however, didnot result in a substantial increase of the spans of concrete bridges.

Some progress with respect to an increase of the Span and the reductionof the Weight was attained by the proposition to place the re-enforcingbars outside of the cross-section of the concrete beam proper, and tosubject them by special means to a using a specially tough steel.Constructions of this kind involve the disadvantage that theconcentrated tension set up in the steel bars must be guided aroundcorners and taken up by anchoring means. Such tensional forces must beguided through points of the beam subject to but comparatively smallbending torques, e. g. through the cross-section of zero torque.Therefore, the re-enforced concrete structure must be dimensioned atsuch points to take up much higher forces than would result from theload. The result is a high dead load limiting the spans for economicalreasons.

Other designers consider it a drawback that, with the above statedmethod, the advantages resulting from the so-called bonding effectbetween the biassed steel bars and the concrete element is notattainable in each crosssection thus requiring a higher bias tocompensate the effects of shrinking and creeping. Such designers use abias combined with a bonding effect, that is to say, they subject thesteel bars to tension in a so-called tensioning bed and then embed thebiassed bars over their entire lengths within the concrete. Because ofthe extreme bias required, complicated and expensive implements must beused for producing it. Using such a method on bridges having severalspans would require a sub-division of the structure into a plurality ofelements and a subsequent assembly of such elements by means of a ratherinvolved constructing process. It is the object of the presentinvention, to provide a pluralspan concrete bridge free from the abovediscussed shortcomings of the prior designs and having substantiallyincreased spans permitting at the same time of a simple, economical andinexpensive way of construction.

According to the present invention this object is attained by thecombination of the following features: The main supporting elements aredesigned as cantilevers freely extending from each of the pillars inboth directions, the opposed ends of adjacent cantilevers being sep- Thespan of such bridges is and, therefore, permitting of a slight mainbeams in a brace-like manner f high tensional force ,coating there-enforcing bars by a arated by a gap. The cantilevers are constructedas trusses of re-enforced concrete. Such concrete trusses are built upby progressively forming successive truss sections starting from thepillars without the use of any scaffold supported on the ground, there-enforcing tension-bars being anchored by the use of elements asordinarily employed in the steel building art.

In a preferred embodiment of the present invention the cantilevertrusses are subject to negative bending torques preferably over theirentire lengths as distinguished from a limited zone above the pillars.This effect is attained for instance with a brige extending over threespans by the provision of a separating gap in the middle of the centralspan, means being provided for ltransferring transverse forcesexcluding, however, the

transfer of any torques or longitudinal forces. In such a case the spansof a three-span bridge are preferably chosen at a ratio of 1:2:1 thusarriving at cantilever arms of equal lengths extending from each of thetwo pillars. With a larger number of spans a similar arrangement may beprovided. This provision, however, is not apt by itself to solve theproblem with respect to an economical method of construction. Thisobject is attained by the design of the cantilever as a truss and by aspecial method of its construction.

The present invention relates more particularly to two alternativespecies of the novel method of constructing the trusses progressively.With the first method the truss beams subject to pressurehereinaftercalled thrust-beams-are formed by the usual process used in the art ofconcrete structures, whereas the truss beams subject totension-hereinafter called tension-beamsare constructed by the methodsused in the steel building art, the re-enforcing bars for thetension-beams and the thrust-beams being rst placed in position in aslack condition whereupon the thrust-beams are formed of concrete. Thenthe re-enforcing bars for the tensionbeams are subjected to tension byremoval of the scaiolds and are then surrounded by and embedded inconcrete.

ln the other species of the novel method of building up the cantilevertrusses progressively section by section, both, the tension-beams aswell as the thrust-beams of the frame-work are formed in accordance withstandard principles used in re-enforced concrete construction, apressure being set up, however, in the concrete element of the tensionbeams by means of a tensional bias produced in the re-cnforcing element,the specific pressure so set up in the concrete being substantially ofthe same magnitude as the specic pressure prevailing in the thrust-beamsof the frame-work. To this end, the reenforcing bars consisting of amaterial having a high tensile strength are prevented from entering intoa bonding relationship to the surrounding concrete either by plastic orby forming the concrete beam with longitudinal bores or cavitiesaccommodating the reenforcing bars in spaced relationship to theconcrete permitting of a free relative longitudinal displacement. Afterthe concrete has set, the re-enforcing bars extending through the beamare subjected to a powerful bias by use of known implements, thereactionary force of the tension set up in the reenforcing bars servingto compress the surrounding concrete lengthwise. Subsequently, a bondmaybe produced between the steel bars and the surrounding concreteelement of the-thrust-beams by `lling up the bores or cavities withcement.

Important advantages in various regards are attained by the presentinvention.

Owing to the provision of a gap in the center, the bending torquesapproach zero with decreasing distance from the gap permitting of aparticularly light design of the adjacent zones of the adjoining trussesas is desir.-

Vthe truss a' continuous one.

Y semifinished condition in which ableV on account of their largedistance from the pillars, whereas a maximum bending torque resultingfrom the own weight of the bridge and largely fluctuating torques fromthe traic load would result within this zone, were The use of a truss ofthe described design permits of taking up the torque prevailing at anypoint with the largest vertical dimension of the trussavailable at suchpoint.Y This again entails the admissibility of an extremely lightdesign of the beams over a large range of the truss. While it is truethat the bending torques prevailing in the truss within the Zone abovethe pillars will exceed those set up in a continuous beam of thecustomary design, 'this consequence is more than olf-set by theafore-stated advantage regarding the bending torques produced. Thesavings in weight of th'ela'rge middle span result in a surprisingincrease of the span possible with a given vertical dimension of thebridge above the pillars, such increase amounting 'to about 35%. With agiven span the present invention makes it possible to reduce thedimensions of the bridge thus resulting in savings of concrete VandVsteel enhancing the eliiciency. The reduction of the materials requiredis not purchased at the cost of complicated and expensive constructionimpiements and building methods for producing the beams. Regarding thedic'ulties involved, the new construction methods do not diiier fromthose ordinarily used in the normal re- 'enforced concrete constructionar The elimination of a scaffold supported by the ground offers greatadvantagesin the construction of bridges of large span, particularly insuch cases in which such scafolds are hampering navigation on rivers orare endangered by lioods and ice. The novel method of constructing thetrusses section by section progressively simplilies the processsubstantially resulting in arcost reduction. This holds true for bothspecies of the novel .constructing method in which one and the samescalold and the same concrete forms supported by the finished trusssections are used requiring a mini um of capital investment for theconstructing implements.

The second species of the constructing method as above described entailsthe advantageous possibility of adjusting the Vbias set up inre-enforcing bars, as the work proceeds, to meet the requirementsarising in the course of the completion ofV the truss. Such'adjustrnentstake care of the fact that the distribution of the forces yset up in thebeams changes with the progress of the work, since in the iinished trusssuch distribution is quite diterent from that prevailing in the initialstages when the truss is in a semi-finished condition and has not yetreached its full length.

Another object of the present invention is a displaceable scaioldcomprising steel beams imposed on the iinished joints of the upper chordand projecting therefrom a distance corresponding to one section, suchscatold being to support the concrete forms so dimensioned as' requiredand the load t'o be taken up in section.

Further objects and features of the present invention and the manner inVwhich the same may be performed as well as the advantages attained bythe invention will appear from a detailed description ot two preferredembodiments of the invention described hereinafter with reference tothe' accompanying drawings.

In the drawings: Y

Fig'. l is an elevation of the 'novel bridge comprising three spans;

Fig. V2 is a partial vertical cross-section through a cantilever ltrussforming part of the bridge said crosssection being take-n within themiddle span at a distance from a pillar or" about one quarter of thespan and shown on an enlarged scale;

Fig. 3 is a partial view of Fig. l on an enlarged scale showing thecentral joint of the two cantilever trusses;

Fig. 4 is an. elevation of one section of the truss in a there-entorcing bars the assembly of one truss s shown in Figs. 4 and 5 arenot visible in Fig.

section through a modisecond species of the placeable scaffold andconcrete form used to successive-` ly construct one section after theother;

Fig. 9 is an end view from the right of Fig'. 8.

The bridge shown in its entirety in Fig. ll embodying the presentinvention extends over three spans A, B and C at a ratio of l:2l. ltcomprises vtwo cantilever trusses carried by and projecting equaldistances from the two pillars 2 and 3. The adjacent ends of thecantilever arms S and 6 are separated by a gap 1. The cantilever arms 4and 7 constituting the spans A and C are supported on embankment restsor foundations 8 and 9 by customary supporting means such as rollerbearings and counterweights or. other means not shown in detail adaptedto transfer forces in a vertical direction.

Each of Sthe two cantilever trusses of identical design comprises anupper chord in form of a plate or reenforced concrete constituting theroad bed, a lower chord of re-enfo'rced concrete preferably in form of aplate, vertical beams between the two chords and diagonal beams adaptedto take up tensional forces.

taken through the truss shownv With this Varrangement the own weight ofthe structur'e produces negative bending torques only in each of thetrusses over their entire lengths, whereas the traffic load producesnegative torques only in the middle span, i. e. in the cantilever arms 5and 6 and positive torques in the outer spans, i. e. in the cantileverarms 4 andV 7. Such positive turques, however, are small compared withthe negative torque caused by the weight of the structure and maysurpassV the same only in the immediate proximity tothe embankment restsand 9. In Figs. 2, 3, 4 and 5 details of the structure are shown toillustrate the cantilever truss composed of thrust-beams built up inaccordance with the standard practice 'of reenforced concreteconstruction and of tension-beams built in accordance with the standardsteel construction practice. Fig. 2 illustrating a cross-section throughthe middle span taken at a distance of about one quarter of the spanfrom the pillar, shows the lower chord 10 constituted by a standardplate of re-enforced concrete and the upper chord 11 re-enforced bybundles of steel bars 12 in cross-section. These bundles take up a spaceof about half of the cross-section of the upper chord the latter formingthe road bed. Moreover, Fig. 2`shows a vertical thrust beam which, inthe present embodiment, is constituted by a plurality of posts orcolumns i3 of re-enforced concrete. The diagonal tension-beamsl 14 2 asthey are hidden from View by the posts V13. Y

Fig. 3 shows the joint between the two cantilever trusses with a gap 1provided therebetween in the middle of the spain B and with a verticallink 15 of known design for the transfer of vertical forces only.

Fig. 4 shows a section of the frame-work truss in. a semi-finishedcondition in which the vertical beams 13 and the lower chord section1l), all subject to thrust, have been completely formed of re-enforcedconcrete, the scailolding and the concrete forms having beeny removed,whereas the re-entorcing bars. 12 of the upper chord 11 and there-enforcing bars of the diagonal beams 14, all subject to tension underthe weight of the semi-finished truss-sections, have not yet beenprovided with a mantle of. cement. It will be appreciated that owing tothe i'es u moval of the scatiolding the thrust beams l!) and 11 as Wellas the tension-bars 12 and i4 are placed under load.

Now the concrete forms are brought into position as will be describedlater for the purpose .Q mantling the tension-bars 12 and 14 withconcrete. Fig. 5 shovf; the finished structure in section illustratingthe re-enforcing bars embedded in concrete.

In a modified process the number of re-enforcing bars placed in positionin a slack condition for the upper chord sections is limited to thefigure required to the weight of the bridge. After assen 13 in thecondition shown in Fig. 4, additional bars or cables sheathed with sheetmetal or a plastic coat adapted to carry the load of the bridge in itsfinished condition are inserted in the concrete form for the upper chordand are then embedded in the concrete. After the latter has set` theadditional re-enforcing bars or cables are subjected to a bias of such adegree in the surrounding concrete is suiiicient to prevent any tensionfrom being exerted on the concrete of the upper chord section under theiniiuence of the biggest possible load of the finished bridge.

This process has the advantage that the road bed will have an increaseddensity and that cracks are prevented from forming under load. Anotheradvantage is the reduction of the number et sleeve joints and otherconnecting elements required to connect the tension-bars of adjoiningupper chord sections. Each of the biassed tension-bars is anchored atits two ends only, that is to say at two points and not at any beamjunction.

The tension-bars may be arranged and distributed in various manners.They may be arranged in a bundle located in a special zone or they maybe distributed over the entire width of the upper chord section. Thehollow space between each of the biassed tension-bars or cables and thesurrounding sheathing may be filled up with cement.

In building up a bridge constituted by cantilever trusses as shown inFigs. l to 5 the following method may be employed:

First two sections of the truss above each of the two pillars 2 and 3are constructed in accordance with the standard practice using thecustomary scaffolding and concrete forms except for the tension-beamswhich are left in semi-iinished condition as shown in Fig. 4. The i theauxiliary scatold shown in Figs. 8 and 9 is brought into position forthe purpose of constructing the adjoining sections progressively. Thisscaffold consists of a plurality of beams 17 arranged parallel to eachother at a suitable distance and distributed over the entire width tjthe bridge resting on the last two beam-junctions 18 and projectingbeyond the end section a distance corresponding to the length of onesection as shown in Fig. 8. From such beams the concrete forms 19 aresuspended by means of suspending elements 20.

With the aid of this scaffold the lower chord section 21 and thevertical posts 22 positioned thereon are formed of 1re-enforced concreteand the diagonal tension-bars 23 and the upper chord reenforcing bars 24are assembled. The section '21 is formed adjoining f he previouslyfinished section abutting against the latter at the joint 26. Thetension-bars 23 and 24 are connected by means of connecting sleeves 27and 28 with those of the preceding truss section. Care must be taken topostpone the mantling of the diagonal tension-bars 23 and of the bars 24of upper chord with concrete until the entire cantilever arm has beenbuilt up in the described manner up to its end.

The other species of the novel method of constructing the cantilevertrusses will now be described hereinafter with reference to Figs. 6 and7.

Fig. 6 illustrates one section of the cantilever truss diftering fromthat illustrated in Figs. l to 5 by the provision of thrust-beams asdiagonal braces 3? in lieu of the diagonal tension-beams shown in thefirst described embodiment. The road bed plate 32 constituting the upperchord that the resulting pressure set up titl 31 of the truss as w il asthe vertical posts 33 are biassed in a special manner. Only part of thereenforcing bars of the upper chord are anchored at 34 at each beamjunction, while the rest of the re-eniorcing bars 35 extend the wholelength of the bridge. Whenever a truss section has been finished and itsconcrete has set, the short tension-bars of the upper chord are putunder tension. In order to prevent the tension bars from entering into abonding relationship with the surrounding concrete and to thus render asubsequent tensioning possible after the concrete set, the re-enforcingbars to be subjected to a bias are either coated with a plastic orinserted in hollows or cavities or bores provided in the concrete.

Fig. 7 shows clearly that the tension-bars 34 are anchored at thebeam-joints whereas the tension-bars 35 are running clear through theentire length of the bridge. With this mode of construction the trussmay be progressi ely built up section by section with the aid of theauxiliary scaffold shown in Figs. 8 and 9 with slight modiiications tosuit the requirements.

The term re-enforcing bars used in the claims following hereinafter isto be construed as including wires, rods, solid bars, cables or thelike. While my invention has been described hereinabove with referenceto specic embodiments thereof 'i wish it to be clearly understood thatmy invention is in no way limited to the specific features of suchembodiments but is capable of numerous modifications within the scope ofthe appended claims.

What I claim is:

l. A method or" constructing a reinforced concrete cantilever bridgecomprising constructing a center pier, anchoring reinforcing rods forthe tirst vertical center strut and first panel lower chords in saidpier, these being compression members with the lower chord being startedsimultaneously vfrom opposite sides of said pier, supporting the outerends of said lower chord members by reinforcing rods for forming adiagonal strut joined to the upper chord, said diagonal strut and upperchord being tension members, encasing the compression members withconcrete and thereby adding load to pretension said tension members,progressively repeating in the same manner the erection of compressionand tension members panel by panel in opposite directions from the pieruntil mid-span is reached, an-d then encasing said tension members withconcrete.

2. A method of constructing a reinforced concrete bridge as in claim l,further comprising constructing said upper and lower chords as slabsextending the entire width of the bridge.

References Cited in the tiie of this patent UNiTED STATES PATENTS OTHERREFERENCES Civil Engineering Publication, March 1941, vol. 11, No. 3,pages 15G-153.

Engineering News Record, Prestressed Concrete Design, October 18, 1945,pp. 92 and 93.

Engineering News Record, June 12, 1947, pp 112, 113.

